Reverse osmosis (RO) is a common method for the treatment for household drinking water. RO systems are typically used to reduce the levels of total dissolved solids and suspended matter. Reverse Osmosis works by forcing water under pressure through a semi-permeable membrane that removes many chemicals at the molecular level. On private water systems reverse osmosis systems are typically used at the tap (under the sink) as a point-of-use treatment device (POU).

RO membranes a generally made of thin film composite (TFC), cellulose acetate (CA) and cellulose triacetate (CTA). TFC membranes have better durability as well as generally higher rejection rates than CA/CTA membranes. TFC are considered more resistant to bio fouling, high pH and high TDS. CA and CTA membranes have a better ability to tolerate chlorine.

RO units do not typically remove 100 percent of a particular chemical or contaminant but can remove a large percent of some chemicals. The removal efficiency can vary depending on the overall water chemistry and other factors such as water temperature and pressure. Generally RO systems waste about 80 percent of the treated water. For every 10 gallons of water treated 2 gallons are usable and the rest goes down the drain as wastewater. However, consideration has to be given to the variability in the efficiencies different types of RO systems.

RO is often combined with other treatment methods such as granular activated carbon and /or point of entry ion exchange (water softener) to improve the over-all treatment efficiency.

Although RO can remove many microorganisms (such as protozoans) it is not considered an acceptable method for the continuous disinfection of private water systems in Ohio. ROs are not whole-house disinfection devices and have the potential for bio-fouling at the RO treatment membrane. Microbiologically unsafe private water systems need to be treated at the point-of-entry (POE), that is whole house treatment systems.

See ANSI / NSF Standard 058 for a listing of RO units and their removal capabilities.

RO units that are used for removing nitrates should have a thin film composite membrane (TFC). RO units using TFC membranes can reduce nitrate concentration by 60 to 95 percent. When nitrate levels exceed 30 mg/l RO becomes less effective and other alternative treatment systems should be considered. RO units that use cellulose acetate membranes is not recommended for nitrate removal private water system. For more information see the nitrate treatment fact sheet for additional information.

RO can remove 60 to 90 percent of arsenic from water depending on the valence of the arsenic (As III or As V). Trivalent arsenic is generally more difficult to remove from drinking water than pentavalent arsenic. Trivalent arsenic can be converted to pentavalent arsenic in the presence of an effective oxidant such as free chlorine. The arsenic in water containing detectable free chlorine or that has been treated with another effective oxidant will be primarily in the pentavalent arsenic form.

If the arsenic valence is unknown, RO may still be used to treat the drinking water at the tap water for arsenic not exceeding 70 ug/l (same as a ppb) with caution. Since arsenic treatment of the water for the trivalent form is estimated to provide 60 percent removal this can reduce the finished water to the US EPA MCL of 10 ug/l for arsenic.

RO is used on some municipal water supplies for desalinization and can have the same application for private water systems to treat high saline (high TDS) water as whole- house treatment system. However, although whole-house RO systems are available, they can be expensive and, in many cases not practical due to the high water use and rejection rate.